Surface treatment for a wrought copper foil for use on a flexible printed circuit board (FPCB)

ABSTRACT

A surface treatment for a wrought copper foil relatives to a surface treatment procedure for a wrought cooper foil for use on a flexible PCB. The surface treatment comprises the steps of bi-polar electrochemical degreasing, acid etching and nodularization, electroplating a barrier layer and an anti-tarnish layer and coating a silane coupling agent layer to a copper foil. One side of the copper foil is nodularization, and a Zn—Ni barrier layer and a Cr—Zn anti-tarnish layer are electroplated on the copper foil. Finally, the organic silane coupling agent is coated to the roughened surface of the copper foil to modify the surface characters to impart the peel strength. Whereby, the cooper foil has very low profile, high peel strength, finer-pattern property, low HCl undercut and high thermal oxidation property.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a surface treatment for a wrought copper foil for use on a flexible printed circuit board (FPCB), and more particularly to a surface treatment that makes the copper foil with low profile and high peel strength.

[0003] 2. Description of Related Art

[0004] Before wrought copper foil for use on a flexible PCB, the copper foil has to be degreased, acid-etched and, nodularized and plating with barrier layer, anti-tarnish layer and coating with a silane coupling agent to satisfy the necessary characteristics to the copper foil for use with a flexible PCB. When treating the copper foil so it can be laminated with a PI (polyimide) film for flexible PCB, the copper foil must have the following characteristics.

[0005] (1) The copper foil must have anti-oxidation and anti-wrinkling properties when the copper foil is stored or transported.

[0006] (2) The copper foil must have an anti-oxidation property, anti-eroding property with regard to chemical additives, non-breaking property and excellent bonding strength when the copper foil is laminatied with the PI film of the flexible PCB.

[0007] (3) The copper foil must have good etching and dielectric properties when the copper foil is etched.

[0008] (4) The copper foil must combine well with the photoresist during the exposure process and be easily cleaned by washing. Additionally, the etched lines on the copper foil cannot be eroded by hydrochloric acid.

[0009] (5) The copper foil must not generate pink ring during thermal shock when soft welding and drilling holes on the flexible PCB.

[0010] The copper foil has a roughened side and a glossy side. The roughened side is laminated with the PI film, and the glossy side is exposed to the photoresist. Therefore, the roughened side and the glossy side must have different characteristics.

[0011] (1) With regards to the roughened side: the surface can have no oxidation or surface defects after laminating the copper foil and the PCB and etching the copper foil. Moreover, the surface can have no residual dust on the surface of the copper foil after etching.

[0012] (2) With regards to the glossy side: the glossy side cannot be oxidized when the copper foil and the PI film are laminated, and the glossy side must combine well with the photoresist, and the shining side must have good solderability.

[0013] The largest difference between the characteristics of the roughened side and the glossy side is that the roughened side of the copper foil has to be nodularuzated before being laminated with the PI film to impart the peel strength. In the nodularization process, the copper foil has to be treated by plating fine copper particles to the surface. Additionally, barrier layer must be plated to prevent erosion by the resin during the laminating process.

[0014] With regard to a conventional electrochemical degreasing process, the copper foil must contact the electrode so achieving the copper foil characteristics previously mentioned is difficult. The electrochemical acid etching process takes a long time and causes too many copper ions to accumulate in the etching solution. Additionally, other conventional surface treatment procedures make the copper foil excessively rough and easily torn.

[0015] To overcome the shortcomings, the present invention provides a surface treatment for a wrought copper foil for use on a flexible PCB to mitigate or obviate the aforementioned problems.

SUMMARY OF THE INVENTION

[0016] The main objective of the invention is to provide a surface treatment for a wrought copper foil for a flexible PCB, which makes the copper foil have a very low surface roughness, excellent peel strength, heat resistance and resistance for the hydrochloric acid. In other words, the copper foil possesses excellent characteristics for a copper foil adapted to laminate with PI film for a flexible PCB.

[0017] Objectives, advantages and novel features of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018]FIG. 1 is a block diagram of a surface treatment for a wrought copper foil for use on a flexible PCB in accordance with the present invention;

[0019]FIG. 2 is a schematic diagram depicting the surface treatment for a copper foil in FIG. 1;

[0020]FIG. 3 is a schematic diagram of the bi-polar electrochemical degreasing process in FIG. 1;

[0021]FIG. 4 is a schematic diagram of the bi-polar electrochemical acid etching process in FIG. 1;

[0022]FIG. 5 is a schematic diagram of the two-stage electrochemical nodularization process in FIG. 1;

[0023]FIG. 6 is a schematic diagram of the barrier layer electroplating process in FIG. 1;

[0024]FIG. 7 is a schematic diagram of the anti-tarnish layer electroplating process in FIG. 1; and

[0025]FIG. 8 is a schematic diagram of the silane coupling agent coating process in FIG. 1.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

[0026] With reference to FIGS. 1 and 2, a surface treatment for a wrought copper foil for use on a flexible PCB comprises the steps of bi-polar electrochemical degreasing, acid etching and nodularization, electroplating a barrier layer and an anti-tarnish layer and coating with silane coupling agent g an organic silane layer.

[0027] With further reference to FIG. 3, copper foil is drawn off from a source roll (12) by a drive roll (16), passes between another drive roll (16) and a squeegee rool (15) and is fed to an electro-cleaning tank to perform the bi-polar electrochemical degreasing of the copper foil. The bi-polar electrochemical degreasing is performed with two electrodes respectively at the current inlet and the current outlet with DC power provided from a controllable DC power source (23). Each electrode is a pair of stainless steel sheets (20). The copper foil passes between the two stainless steel sheets (20) of the positive charge at the current inlet, around a sinking roll (14) and between the two stainless steel sheets (20) of the negative charge at the current outlet. The bi-polar electrochemical degreasing process utilizes a cleaning solution containing sodium hydroxide, an alkaline metal carbonate, an alkaline metal silicate as the electrolyte. The current density in the range from about 1˜5 A/dm² for about 10˜40 seconds at a temperature of from about 50˜80° C. The advantages of using the bi-polar degreasing process are that the copper foil does not contact with the conductive roll and particle is not deposited on the copper foil at the current outlet because the copper foil at the current outlet is anode. After the copper foil leaves the electro-cleaning tank, a drive roll (16) and a squeegee roll (15) squeegee excess solution from the copper foil before rinsing the copper foil twice with hot water in a rinsing tank (10).

[0028] With further reference to FIG. 4, the copper foil passes between a drive roll (16) and a squeegee roll (15) and is fed to an etching tank to perform the bi-polar electrochemical etching of the copper foil. The bi-polar electrochemical etching is performed with two electrodes respectively at the current inlet and the current outlet with DC power provided from a controllable DC power source (23). The bi-polar electrochemical acid etching is performed by passing the copper foil between two pairs of electrodes in a sulfuric acid solution. The electrodes in each pair are Pt/Ti sheets (21) and are oppositely charged. The copper foil passes through the sulfuric acid solution between the first pair of Pt/Ti sheets (21) of negative charge at the current inlet, around a sinking roll (14) and between the second pair of Pt/Ti sheets (21) of positive charge at the current outlet. The electro-etching utilizes a solution contain sulfuric acid and copper ions. The sulfuric acid is present in an amount from about 30˜90 g/L. DC current source (23) maintains a current density in the range from about 2˜10 A/dm² for about 5˜20 seconds at a temperature of from about 30˜60° C. The advantages of using the bi-polar electrochemical etching process are that the copper foil does not contact other copper foil and copper ions do not accumulate on the copper foil in the etching solution. After passing between the second pair of the Pt/Ti sheets (21) and out of the etching tank, the copper foil is squeezed between a drive roll (16) and a squeegee roll (15) before being cleaned with deionized water in a rinsing tank (10).

[0029] With reference to FIG. 5, the nodularization process is performed by passing the copper foil through a first nodularization tank and a second nodularization tank containing an electrolytic solution. In the first nodularization tank, the copper foil is connected to a negative electrode of a DC power source (23) through a conductive roll (17). The copper foil passes through the electrolytic solution in close proximity to a positively charged electrode (22), around sinking roll (14) and out of the tank between a drive roll (16) and a squeegee roll (15). In the first tank, the copper foil is nodularized by DC plating with high-density current to nucleate copper micro-particles on the surface of the copper foil facing the positive electrode. The nodularization process utilizes a solution sulfuric acid, copper ions and additives. The tank is carried out in an electrolytic solution containing 50˜150 g/L sulfuric acid, 5˜20 g/L copper ions and additives. The copper foil is a cathode and the DSA (Dimension Stable Anode) becomes an anode. The current density in a range from about 60˜150 A/dm² for from 1˜3 seconds at a temperature of from about 20˜40° C. When the copper foil leaves the first nodularization tank, residual solution is squeezed from the copper foil by a drive roll (16) and a squeegee roll (15) and passes through a rinsing tank (10) before moving into a second nodularization tank.

[0030] In the second nodularization tank, the copper foil is also connected to the negative electrode of a DC power source (23) through another conductive roll (17) and passes in close proximity to a positive charged electrode (22) in the second nodularization tank, around a sinking roll (14), out of the tank between a drive roll (16) and a squeegee roll (15) and through a rinsing tank (10). In the second nodularization tank, the copper foil is nodularized by direct current plating and treated with low-density current to growth the copper nodular on the surface of the copper foil. The nodularization process in the second nodularization tank is carried out in the same solution in the first phase. The current density in a range from about 10˜60 A/dm² for from 4˜10 seconds at a temperature of from about 20˜40° C. Additionally, additives containing As⁺⁵, As⁺³, Ni⁺², W⁺⁶, NO₃ ⁻, Co⁺² ions are added to the plating solution to inhibit the copper deposition. The additive is present in the plating solution in the range from about 0.2˜2.0 g/L, and preferably from about 0.5˜1.5 g/L of As⁺⁵. When the copper foil leaves the second nodularization tank, a drive roll (16) and a squeegee roll (15) squeegee residual solution from the copper foil, and the copper foil is further cleaned with deionized water in a rinsing tank (10).

[0031] With reference to FIG. 6, the Zn—Ni barrier layer is applied to the copper foil utilizes a solution containing the zone ion, nickel, boric acid, sodium sulfate as the electrolyte in the tank. The copper foil is channeled into the g tank between a conductive roll (17) and a squeegee roll (15) at an inlet and leaves the tank between a conductive roll (17) and a squeegee roll (15) at an outlet. The copper foil is connected to negative electrodes of three DC current sources (23) that are connected to the two conductive roll (17) at the inlet and the outlet of the tank. In the plating solution in the Zn—Ni alloy plating tank, the copper foil passes between two positively charged Pt/Ti electrodes (21) at the inlet, around a sinking roll (14) and in close proximity to another positively charged Pt/Ti electrode (21) at the outlet. Because the copper foil is negatively charged in the tank, a barrier layer is directly plated on the copper foil. The barrier layer plating process is carried out in a 20˜100 g/L boric acid solution containing 0.25˜2.0 g/L Ni ions and 0.5˜5.0 g/L Zn ions. The current density in the range from about 0.5˜2.0 A/dm² for from 2˜10 seconds at a temperature of from about 30˜60° C. An additive selected from saccharine or benzyl tri-ethyl ammonium bromide is added to the solution in a range from about 50˜300 ppm. The pH of the plating solution is maintained at a range from about 4.0˜6.0. When the copper foil leaves the tank, the conductive roll (17) and the squeegee roll (15) squeegee residual solution from the copper foil before rinsing the copper foil with deionized water in a rinsing tank (10).

[0032] With reference to FIG. 7, an Cr—Zn anti-tarnish layer is applied to the copper foil utilizes a solution contating chromic acid, zinc ion, sodium sulfate as the electrolyte in the tank. The copper foil is channeled into the tank between a conductive roll (17) and a squeegee roll (15) at an inlet and leaves the plating tank between a conductive roll (17) and a squeegee roll (15) at an outlet. The copper foil is connected to negative electrodes of two DC current sources (23) that are connected to the conductive roll (17) at inlet of the anti-tarnish layer plating tank. The copper foil passes between two positively charged Pt/Ti electrodes (21) at the inlet, around a sinking roll (14) and out of the tank. Because the copper foil is negatively charged in the tank, an anti-tarnish layer is directly plated on the copper foil. The anti-tarnish layer plating process is carried out in a 2˜20 g/L sodium sulfate solution containing 0.2˜2.0 g/L Zn ions and 0.5˜5.0 g/L Cr acid. The current density in the range from about 0.5˜5.0 A/dm² for from 2˜10 seconds at a temperature of from about 30˜60° C. When the copper foil leaves the g tank, a drive roll (16) and a squeegee roll (15) squeegee residual solution from the copper foil before rinsing the copper foil with deionized water in a rinsing tank (10).

[0033] With reference to FIG. 8, the roughened surface is coated with a silane coupling agent to form a thin film on the copper foil in a tank. The copper foil is fed through the tank around a sinking roll (14), between a drive roll (16) and a squeegee roll (15) to a drying oven (11). The tank holds a solution containing 0.2˜2.0% 3-glycidoxypropyl trimethoxy silane to coat the silane coupling agent on the copper foil. Only the roughened side of the copper foil is coated with the silane coupling agent. After the roughened side of the copper foil is coated with the silane coupling agent layer, the copper foil leaves the tank, and a drive rool (16) and a squeegee roll (15) squeegee excess silane coupling agent from the copper foil before the copper foil is dried in the drying oven (11). Finally, the treated copper foil is rolled onto a copper roll (13).

[0034] Several examples of the surface treatment of a copper foil for a flexible PCB to illustrate different operational conditions used in the performance of the process follow.

EXAMPLE 1

[0035] 35 μm thick wrought copper foil was electrochemically degreased using a bi-polar setup in a solution of a 10% base degreasing agent at 70° C. with a current density of 2 A/dm² for 10 sec.

[0036] The copper foil was then electrochemically etched using a bi-polar setup in a 45 g/L concentration of sulfuric acid at 45° C. with a current density of 2 A/dm² for 10 sec.

[0037] One side of the copper foil was nodularized in two plating stages having different current densities. The first nodularization plating stage was performed in a solution with a copper concentration of 14 g/L, a sulfuric acid concentration of 90 g/L and an As⁺⁵ concentration of 1.45 g/L at 30° C. with a direct current density between the electrodes of 131 A/dm² for 1.0 sec. The second nodularization plating stage was performed with a current density of 20A/dm² for 5 sec.

[0038] The roughened side of the copper foil was coated with a barrier layer by electroplating. The barrier layer plating was performed in a solution with a Zn concentration of 2.0 g/L, a Ni concentration of 0.75 g/L and an H₃BO₃ concentration of 40 g/L at 50° C. with a current density of 0.75 A/dm² for 10 sec. The current density on the glossy side of the copper foil was plated with current density of 0.2 A/dm² for 2.5 sec.

[0039] An anti-tarnish layer was plated on both surfaces of the copper foil in a solution with a CrO₃ concentration of 1.6 g/L, a Zn concentration of 0.5 g/L and a Na₂SO₄ concentration of 10 g/L at 35° C. with a current density of 2 A/dm² for 5 sec.

[0040] An silane coupling layer was coated to the roughened side of the copper foil by spraying with a 0.5% 3-glycidoxypropyl trimethoxy silane solution. Then FR-4 was laminated with copper foil. Finally, the laminated copper foil was etched with cupric chloride and subjected to a peel strength test.

EXAMPLE 2

[0041] 35 μm thick wrought copper foil was electrochemically degreased using a bi-polar setup in a solution of a 10% base degreasing agent at 70° C. with a current density of 2A/dm² for 10 sec.

[0042] The copper foil was then electrochemically etched using a bi-polar setup in a 45 g/L concentration of sulfuric acid at 45° C. with a current density of 2 A/dm² for 10 sec.

[0043] One side of the copper foil was roughened in two plating stages having different current densities. The first nodularization plating stage was performed in a solution with a copper concentration of 14 g/L, a sulfuric acid concentration of 90 g/L and an As⁺⁵ concentration of 1.45 g/L at 30° C. with a direct current density at the inlet of 131 A/dm² for 1.0 sec. The second nodularization plating stage was performed with a current density of 20 A/dm² for 5 sec.

[0044] The roughened side of the copper foil was coated with a barrier layer by electroplating. The barrier layer plating was performed in a solution with a Zn concentration of 2.0 g/L, a Ni concentration of 0.75 g/L and an H₃BO₃ concentration of 40 g/L at 50° C. with a current density at the inlet of 0.75 A/dm² for 10 sec.; The current density on the glossy side of the copper foil was plated with a current of 0.2 A/dm² for 2.5 sec.

[0045] An anti-tarnish layer was plated on both surfaces of the copper foil in a solution with a CrO₃ concentration of 1.6 g/L, a Zn concentration of 0.5 g/L and a Na₂SO₄ concentration of 10 g/L at 35° C. with a current density of 1.8 A/dm² for 5 sec.

[0046] A silane coupling layer was coated to the roughened side of the copper foil by spraying with a 0.5% 3-glycidoxypropyl trimethoxy silane solution. Then FR-4 was laminated with copper foil. Finally, the laminated copper foil was etched with cupric chloride and subjected to a peel strength test.

EXAMPLE 3

[0047] 35 μm thick wrought copper foil was electrochemically degreased using a bi-polar setup in a solution of 10% base degreasing agent at 70° C. with a current density of 2A/dm² for 10 sec.

[0048] The copper foil was then electrochemically etched using a bi-polar setup in a 45 g/L concentration of sulfuric acid at 45° C. with a current density of 2 A/dm² for 10 sec.

[0049] One side of the copper foil was nodularized in two plating stages having different current densities. The first nodularization plating stage was performed in a solution with a copper concentration of 14 g/L, a sulfuric acid concentration of 90 g/L and an As⁺⁵ concentration an 1.45 g/L at 30° C. with a direct current density at the inlet of 90 A/dm² for 1.5 sec. The second nodularization plating stage was performed with a current density of 20 A/dm² for 5 sec.

[0050] The roughened side of the copper foil was coated with a barrier layer by electroplating. The barrier layer plating was performed in a solution with a Zn concentration of 2.0 g/L a Ni concentration of 0.75 g/L and an H₃BO₃ concentration of 40 g/L at 50° C. with a current density at the inlet of 0.87 A/dm² for 10 sec. The current density on the glossy side of the copper foil was plated with a current density of 0.2A/dm² for 2.5 sec.

[0051] An anti-tarnish layer was plated on both surfaces of the copper foil in a solution with a CrO₃ concentration of 1.6 g/L, a Zn concentration of 0.5 g/L and a Na₂SO₄ concentration of 10 g/L at 35° C. with a current density of 2 A/dm² for 5 sec.

[0052] A silane coupling layer was coated to the roughened side of the copper foil by spraying with a 0.5% 3-glycidoxypropyl trimethoxy silane solution. Then FR-4 was laminated with copper foil. Finally, the laminated copper foil was etched with cupric chloride and subjected to a peel strength test.

EXAMPLE 4

[0053] 35 μm thick wrought copper foil was electrochemically degreased using a bi-polar setup in a solution of a 10% base degreasing agent at 70° C. with a current density of 2 A/dm² for 10 sec.

[0054] The copper foil was then electrochemically etched using a bi-polar setup in a 45 g/L concentration of sulfuric acid at 45° C. with a current density of 3 A/dm² for 10 sec.

[0055] One side of the copper foil was roughened in two plating stages having different current densities. The first nodularization stage was performed in a solution with a copper concentration of 14 g/L, a sulfuric acid concentration of 90 g/L and an As⁺⁵ concentration of 1.45 g/L at 30° C. with a direct current density at the inlet of 90 A/dm² for 1.5 sec. The second nodularization plating stage was performed with a current density of 20 A/dm² for was 5 sec.

[0056] The roughened side of the copper foil was coated with a barrier layer by electroplating. The barrier layer plating was performed in a solution with a Zn concentration of 2.0 g/L, a Ni concentration of 0.75 g/L and an H₃BO₃ concentration of 40 g/L at 50° C. with a current density at the inlet of 0.87 A/dm² for 10 sec. The current density on the glossy side of the copper foil was plated with a current density of 0.2A/dm² for 2.5 sec.

[0057] An anti-tarnish layer was plated on both surfaces of the copper foil in a solution with a CrO₃ concentration of 1.6 g/L, a Zn concentration of 0.5 g/L and a Na₂SO₄ concentration of 10 g/L at 35° C. with a current density of 2 A/dm² for 5 sec.

[0058] A silane coupling layer was coated to the roughened side of the copper foil by spraying with a 0.5% 3-glycidoxypropyl trimethoxy silane solution. Then FR-4 was laminated with copper foil. Finally, the laminated copper foil was etched with cupric chloride and subjected to a peel strength test.

EXAMPLE 5

[0059] 35 μm thick wrought copper foil was electrochemically degreased using a bi-polar setup in a solution of a 10% base degreasing agent at 70° C. with a current density of 2 A/dm² for 10 sec.

[0060] The copper foil was then electrochemically etched using a bi-polar setup in a 45 g/L concentration of sulfuric acid at 45° C. with a current density of 2.2 A/dm² for 10 sec.

[0061] One side of the copper foil was nodularized in two plating stages having different current densities. The first nodularization lating stage was performed in a solution with a copper concentration of 14 g/L, a sulfuric acid concentration of 90 g/L and an As⁺⁵ concentration of 1.45 g/L at 30° C. with a direct current density at the inlet of 90 A/dm² for 1.5 sec. The second nodularization plating stage was performed with a current density 20 A/dm² for 5 sec.

[0062] The roughened side of the copper foil was coated with barrier layer by electroplating. The barrier layer plating was performed in a solution with a Zn concentration of 2.0 g/L, a Ni concentration of 0.75 g/L and an H₃BO₃ concentration of 40 g/L at 50° C. with a current density at the inlet of 0.75 A/dm² for 10 sec. The current density on the glossy side of the copper foil was plated with a current density of 0.2A/dm² for 2.5 sec.

[0063] An anti-tarnish layer was plated on both surfaces of the copper foil in a solution with a CrO₃ concentration of 1.6 g/L, a Zn concentration of 0.5 g/L and a Na₂SO₄ concentration of 10 g/L at 35° C. with a current density of 2 A/dm² for 5 sec.

[0064] A silane coupling layer was coated to the roughened side of the copper foil by spraying with a 0.5% 3-glycidoxypropyl trimethoxy silane solution. Then FR-4 was laminated with copper foil. Finally, the laminated copper foil was etched with cupric chloride and subjected to a peel strength test.

[0065] The results of the peel strength, HCl undercut, thermal age, and HTO test in the five examples are provided in the following table: Ra Rz Rmax Peel (FR-4) Erosion Proportion Thermal-aged Number (μm) (μm) (μm) (kg/cm) to HCL^(*1) % Proportion^(*2) % Anti-oxidation Test^(*3) Example 1 0.2343 1.757 2.478 1.310 4.10 23.45 Pass Example 2 0.2087 1.524 1.934 1.346 1.23 26.37 Pass Example 3 0.2001 1.353 1.618 1.326 3.54 17.04 Pass Example 4 0.1997 1.321 1.552 1.314 4.87 23.30 Pass Example 5 0.2321 1.616 2.225 1.375 1.83 25.69 Pass

[0066] 1. Resistance to hydrochloric acid was given as deterioation rate (%) of peel strength of a 8 mill pitch circuit measured after immersion in 18% HCL for 1 hour.

[0067] 2. Resistance to thermal deterioration was measured in terms of deterioration rate (%) of the peel strength after heating at 177° C. for 48 hours.

[0068] 3. Testing conditions 270° C., 1 hr.

[0069] The surface treatment for a wrought copper foil for use on a flexible printed circuit board (PCB) as previously described has the numerous advantages over conventional procedures. The degreasing, etching, nodularization and barrier layer and anti-oxidation layer plating steps in the process all have significant advantages.

[0070] The degreasing and etching processes are carried out using bi-polar techniques. The copper foil does not contact the roller. Copper ions do not accumulate on the copper foil during etching processes.

[0071] Carrying out the nodularization process in two plating stages respectively with different current densities prevents wrinkles from forming on the copper foil and enhances the uniformity of the copper foil. If a proper additive is used in the nodularization process, the surface roughness (Rz) is less than 2.3 μm, and the nodularization particles are controlled within 0.5˜0.8 μm. Additionally, the peel strength is larger than 1.3 kg/cm.

[0072] The barrier layer plating process uses Zn—Ni type boric acid as a plating agent. Therefore, the pH is easily maintained, and the copper foil is easily cleaned with water. The anti-tarnish layer plating process uses Cr—Zn type as a plating agent. The surface roughness is reduced, and the peel strength is increased. Furthermore, the HCl undercut of the treated copper foil was less than 5%, the thermal age was less than 30% and HTO at least 1 hr at 250° C. Therefore, the copper foil treated with the surface treatment procedure of the present invention has excellent physical properties to meet the requirements for producing a flexible PCB.

[0073] Even though numerous advantages of the present invention have been set forth in the foregoing description, the disclosure is illustrative only. Changes may be made in detail within the principles of the invention to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed. 

What is claimed is:
 1. A surface treatment procedure of a copper foil for use on a flexible PCB comprising the following sequential acts: electrochemically degreasing a copper foil; acid etching the copper foil; nodularization the roughened side of the copper foil in two stages of different current density; electroplating a barrier layer on the copper foil; electroplating an anti-tarnish layer on the copper foil; and coating an silane coupling agent on the roughened surface of the copper foil; wherein the electrochemically degreasing and acid etching processes are carried out by bi-polar electrochemical processes.
 2. The surface treatment of a copper foil as claimed in claim 1, wherein the bi-polar electrochemical degreasing process is performed in a solution of a base degreasing agent with a current density of 1˜5 A/dm² at 50˜80° C. for 10˜40 sec.
 3. The surface treatment of a copper foil as claimed in claim 1, wherein the bi-polar acid etching process is performed in a solution of 30˜90 g/L sulfuric acid with a current density of 2˜10 A/dm² at 30˜60° C. for 5˜20 sec.
 4. The surface treatment of a copper foil as claimed in claim 1, wherein the nodularization process is performed in two stages where: a first stage is performed just after the copper foil enters a solution of copper ions in a 5-20 g/L plating agent and 50˜150 g/L sulfuric acid and an additive to the plating agent containing As⁺⁵, As⁺³, Ni⁺², W⁺⁶, NO₃ ⁻, Co⁺² with a current density of 60˜150 A/dm² at 20˜40° C. for 1˜3 sec; and a second stage just after the copper foil enters a solution the same as that in the first stage with a current density of 10˜60 A/dm² at 20˜40° C. for 4˜10 sec.
 5. The surface treatment of a copper foil as claimed in claim 4, wherein the additive in the nodularization process contains 0.5˜1.5 g/L of As⁺⁵.
 6. The surface treatment of a copper foil as claimed in claim 1, wherein the barrier layer plating process is preformed in a Zn—Ni type boric acid solution with a 20˜100 g/L concentration of boric acid, 50˜300 ppm of an additive selected from saccharine or benzyl tri-ethyl ammonium bromide, a 0.5˜5.0 g/L concentration of Zn, a 0.25˜2.0 g/L concentration of Ni with a current density of 0.25˜2.0 A/dm² 30˜60° C. for 2˜10 sec and pH in the range of 4.0˜6.0.
 7. The surface treatment of a copper foil as claimed in claim 1, wherein the anti-tarnish layer coating process is performed in a Cr—Zn type solution with a 2˜20 g/L concentration of sodium sulfate, a 0.2˜2.0 g/L concentration of Zn and a 0.5˜5.0 g/L concentration of chromic acid at 30˜60° C. with a current density of 0.5˜5.0 A/dm² for 2˜10 sec.
 8. The surface treatment of a copper foil as claimed in claim 1, wherein the organic silane layer spraying process is performed in a 3-glycidoxypropyl trimethoxy silane solution to coat the silane coupling agent on the roughened side of the copper foil.
 9. The surface treatment of a copper foil as claimed in claim 8, wherein the concentration of the 3-glycidoxypropyl trimethoxy silane solution is 0.2˜2.0%. 